Effectiveness of speed indicator devices: An observational study in South London

► The effectiveness of speed indicator devices on vehicle speeds were evaluated in London. ► Vehicle speeds were reduced on average by 1.4 mph and by 2.6 mph at one site. ► Speed reductions occur up to a distance of 400 m downstream. ► Speed reductions return to normal levels about one week after SIDs are removed. Speed indicator devices (SIDs) are vehicle activated signs displaying the real-time speeds of passing vehicles. They are part of the array of road safety measures for managing speeds on local authority roads in the United Kingdom. This paper documents an evaluation of the effectiveness of SIDs that was carried out in South London in 2008. SIDs were installed at 10 sites in South London for periods of between one and three weeks. The overall effect of the SIDs being installed was a reduction in vehicle speeds of 1.4 mph and a significant reduction in the proportion of vehicles exceeding the speed limit. The research also investigated the distance over which the SIDs are effective and the effect on vehicle speeds once the SID is removed. The results showed that a speed reduction occurred over a distance of up to 400 m and that once the SID is removed then, in general, the mean vehicle speeds returned to pre-SID levels by the end of the first week.